As is well known, a magnet coil can be made superconducting by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel and reducing its temperature to superconducting levels such as 4-10.degree. Kelvin. The extreme cold reduces the resistance of the magnet coil to negligible levels, such that when a power source is initially connected to the coil for a period of time to introduce a current flow through the coil, the current will continue to flow through the coil due to the negligible coil resistance at superconducting temperatures even after power is removed, thereby maintaining a strong, steady magnetic field. Superconducting magnets find wide application, for example, in the field of magnetic resonance imaging (hereinafter "MRI").
In a typical MRI magnet, the main superconducting magnet coils are enclosed in a cylindrically shaped pressure vessel which is contained within an evacuated vessel and forms an imaging bore in the central region. The main magnet coils develop a strong magnetic field in the imaging bore which must be very homogenous and temporally constant for accurate imaging.
Superconducting temperatures are commonly obtained by boiling a liquid cryogen such as liquid helium within the pressure vessel. While the use of liquid helium to provide cryogenic temperatures is widely practiced and is satisfactory for MRI operation, the provision of a steady supply of liquid helium to MRI installations all over the world and its storage and use has proved to be difficult and costly. As a result, considerable effort has been directed at the use of mechanical displacement type cryocoolers or conduction cooling for recondensing the helium gas resulting from the boiling, and then recycling the condensed helium.
However, the ability of mechanical cryocoolers such as the Gifford-McMahon type, to provide the necessary amount and required degree of cooling has often been marginal. As a result, it is highly desirable to minimize the load on the cryocooler and to maximize the capacity of the cryocooler to the extent practical. One type of cryocooler that has desirable cooling capacity uses rare earth materials as the displacement material in the moving piston of the compressor for the second or cold stage of a two stage cryocooler. The rare earth materials such as Er.sub.3 Ni, HoCu.sub.2, or ErNiCo produces relatively high heat capacity in the superconducting temperature range of 4-10.degree. K due to magnetic transitions to enable low temperature operation.
However, cryocoolers utilizing ferromagnetic transition rare earth materials in MRI applications can cause significant distortions and perturbations of the magnetic field in the MRI imaging volume. The rare earth material in the moving displacer is believed to act as a moving magnet of variable field strength when it becomes magnetized by the local field of a typical superconductive magnet, in turn causing magnetic field fluctuations in the imaging volume of the superconductive magnet, which creates unacceptable distortions in the images rendered.
The time varying magnetic fields generated by the moving rare earth displacer can also induce eddy currents in various metallic structures of the main magnet assembly and in the main magnet coils. The presence of eddy currents is most undesirable in MRI applications of rare earth cryocoolers since such eddy currents will generate heat due to the finite electrical resistance of the structures and coils involved. This is the so called AC heating effects of eddy currents which is an additional thermal burden in providing adequate cooling in systems utilizing cryocoolers.
While magnetic shielding arrangements have been used in MRI equipment including bucking coils, and copper and superconducting shields, image quality and other problems discussed above have hindered the commercial use of rare earth cryocoolers in MRI equipment. While superconducting shields have been proposed to surround and shield rare earth cryocoolers, such shields have proven to add considerable cost and weight to the extent that removal of such a cryocooler with an attached shield from an MRI magnet is difficult for one person to physically accomplish. In addition, such shields produce measurable heating due to hystersis and eddy current losses induced by the changing fields, and consume an undesirable fraction of the available low temperature cooling capacity.